The present invention relates to a novel liquid crystal device, a production method therefor, and a liquid crystal alignment control method, as well as to a light-emitting device, such as an LED, and a carrier-transporting device suitable for electrophotography, etc., using the liquid crystal device.
In recent years, carrier-transporting devices, such as semiconductor devices, using organic compounds have been studied. One of the most popular examples thereof is an organic photosensitive semiconductor (OPC) for electrophotography, and it is widely used in electrophotographic copying machines and printers in view of its excellent productivity and functionality. Further, in recent years, extensive research and development has been conducted on the utilization of organic materials also in light-emitting devices such as EL (electroluminescence) devices and LED (light-emitting diode) devices. For example, there are reported a study for remarkable improvement in organic EL device functions by functional separation of a carrier-transporting layer and a light-emitting layer by C. W. Tang, et al. (Appl. Phys. Lett., vol. 51, p. 913 (1987)) and several recent studies showing improved reliability by Pioneer K.K. (e.g., Flat Panel Display, p. 212 (1999) published from Nikkei BP K.K.).
Under these circumstances, there is a trend to use liquid crystal materials in carrier-transporting devices such as semiconductor devices or light-emitting devices such as EL devices and LED devices. This is based on the idea of utilizing a highly ordered liquid crystal structure for preparing high performance devices. For example, Hanna has reported a study on liquid crystalline organic semiconductor materials (Oyo Buturi, Appl. Phys., vol. 68 (1), p. 26 (1999)), wherein a high carrier-transporting characteristic of smectic liquid crystal has been confirmed. Further, A.M. Van de Craats, et al., has also confirmed a high carrier-transporting characteristic of discotic liquid crystal (Adv. Mater., vol. 8, p. 823 (1996)).
Notably, in order to use a liquid crystal in a highly aligned state, the presence of an alignment film (alignment control film) for realizing the aligned state is ordinarily indispensable; however, such an alignment film is liable to obstruct the performances of the above-mentioned type of functional devices. As a result, alignment films have not been used in most of the previous studies, and in some cases where such an alignment film is used, the alignment film is not allowed to directly contact the liquid crystal layer and another layer, typically electrodes. This results in a remarkable decrease in performance, particularly in EL and LED devices. Accordingly, it has become impossible to utilize a liquid crystal in its highly aligned state in most cases. The use of such a highly aligned liquid crystal has been possible by exceptional methods such as shearing a liquid crystal between the substrates or sandwiching a discotic liquid crystal between two substrates. However, the former method has a drawback of poor productivity with regard to reproducibility and the production of large-area devices. The latter method allows only a structure of using two rigid substrates, such as glass plates, and the uniformity of alignment is still insufficient. Further, the method is applicable to discotic liquid crystals only.
Under the circumstances, it has been earnestly desired to develop a method, an organization or a device capable of highly aligning extensive liquid crystal materials inclusive of nematic and smectic liquid crystals that can be provided with a uniaxial alignment characteristic, while obviating a performance obstruction caused by an alignment film. Further, because of such circumstances, the proposal of new functional devices utilizing a highly aligned state of liquid crystal has been severely restricted.
In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a liquid crystal device wherein a high order of liquid crystal alignment state is realized.
Other objects of the present invention are to provide a carrier-transporting device and a light-emitting device utilizing such a liquid crystal device.
According to the present invention, there is provided a liquid crystal device comprising a substrate having an electrode thereon, and a liquid crystal disposed on the substrate, wherein the substrate is provided with portions of an alignment control layer having a uniaxial alignment control power locally formed thereon, whereby the liquid crystal is placed in an alignment state which has been formed through a process of liquid-liquid crystal phase transition on temperature decrease wherein a liquid crystal phase is first generated in regions in contact with the portion of alignment control layer and continuously enlarged in a direction of the uniaxial alignment control power.
The present invention further provides a method for alignment control of a liquid crystal comprising the steps of: providing a liquid crystal device including a substrate having an electrode thereon, and a liquid crystal disposed on the substrate, wherein the substrate is provided with portions of an alignment control layer having a uniaxial alignment control power locally formed thereon, and cooling the liquid crystal from its liquid phase to its liquid crystal phase, thereby generating a liquid crystal phase of the liquid crystal in regions in contact with the portions of the alignment control layer and enlarging the liquid crystal phase regions in a direction of the uniaxial alignment control power to form an alignment state of the liquid crystal phase over the substrate.
According to another aspect of the present invention, there is further provided a method of producing a liquid crystal device including a substrate having an electrode thereon, and a liquid crystal disposed on the substrate comprising the steps of: forming portions of an alignment control layer having a uniaxial alignment control power locally selectively on the substrate, placing the liquid crystal by application or vapor deposition over the portions of the alignment control layer on the substrate, and cooling the liquid crystal from its liquid phase to its liquid crystal phase, thereby generating a liquid crystal phase of the liquid crystal in regions in contact with the portions of the alignment control layer and enlarging the liquid crystal phase regions in a direction of the uniaxial alignment control power to form an alignment state of the liquid crystal phase over the substrate.
The present invention further provides a carrier-transporting device comprising the above-mentioned liquid crystal device, wherein the liquid crystal comprises a liquid crystal having a charge-transporting function in the liquid crystal phase and a light-emitting device comprising the above-mentioned liquid crystal device and including a light-emitting layer.